Radio luminescent sighting arrangement

ABSTRACT

An illuminating arrangement is disclosed which, in one particular embodiment, includes a pair of optically-alignable &#39;&#39;&#39;&#39;sight markings&#39;&#39;&#39;&#39;, the markings being illuminated and defined, at least in part, by a &#39;&#39;&#39;&#39;radio-luminescent&#39;&#39;&#39;&#39; segment, especially suited for a dimly-lit field of view, these segments including a phosphorescent material and a &#39;&#39;&#39;&#39;matched&#39;&#39;&#39;&#39; radio-isotope adapted to induce prescribed phosphorescence.

UnitedStates Patent 1191 James: Dean B. et al. 1 Jan. 8, 1974 [54] RADIOLUMINESCENT SIGHTING 2,488,541 11/1949 Holme 250/71 R ARRANGEMENT1,423,184 7/1922 Butler 250/71 R 1,534,937 4/1925 Foley 250/77 [75]Inventors: Dean B- Ja s, Be y r 2,113,973 4/1938 Addink 250/71 R H.Smiley, Pittsburgh; Ralph E.

l ll g gg Jr Upper St C a Primary Examiner-Archie R. BorcheltAttorney-John R. Eubank and John C. Martin, Jr. [73] Assignee: AtlanticRichfield Company, Glenolden, Pa.

22 Filed: May 11, 1971 [571 ABSTRACT [21] Appl. No.: 142,190 Anilluminating arrangement is disclosed which, in

one particular embodiment, includes a pair of optically-alignable sightmarkings", the markings being illuminated and def-med atvleast in partby a [58] d R 77 78 luminescent segment, especially suited for adimly-lit e 0 field of view, these segments including a phosphorescentmaterial and a matched" radio-isotope adapted [56] ggifi gf to induceprescribed phosphorescence. 3,578,973 5/ 1971v Dooley et al 250/71 R 4Claims, 6 Drawing Figures PATENTEUJAH 8 m4 SHEEI 10F 3 FIGURE I FIGURE 5FIGURE 4 PAIENIEUm 8 m4 3. 784.81 7

sum 2 or 3 FIGURE 3 Pmminm 8mm 3.784.817

saw an: a

FIGURE 5 RADIO LUMINESCENT SIGHTING ARRANGEMENT BACKGROUND OF THEINVENTION When using a sighting arrangement, (such as the sights on arifle, pistol or other weapon) under conditions of low-level (poor)ilumination, an operator (e.g., a rifleman) often is unable to perceivethe sight markings and is thus hampered in the use of the device. Forinstance, when a rifleman is aiming his weapon in the half-light ofnear-dusk or moonlight, he may be able to dimly perceive his target butwill often have difficulty identifying the markings or critical edges ofthe rifle sights. The brightness of the illumination should be justsufficient to provide adequate definition. If it is too bright, theoperator will be blinded; or, at least, his ability to perceive thedimly-lit target will be impaired.

Various arrangements have been considered to illuminate such sightingdevices; for instance, a (miniaturized) battery-powered lamp has beensuggested, preferably in combination with light-conducting fibers(light-pipes) so as to remotely-locate the lamp and completely shieldit, the fibers conducting the light from the lamp to illuminate theeyepiece markings. Such arrangements may use a miniaturized lampbatterycombination, together with a small variable resistance for adjusting thecurrent to the lamp, and resultant brightness e.g., permitting arifleman to illuminate his sights sufficiently to aim his weapon but notso bright as to interfere with his vision, nor to generate glare aboutthe sight markings, or, especially, not such as to signal his presenceto the enemy. The minimal brightness that is required will vary with thedarkness of the surroundings. This battery-powered, lamp-fiberarrangement has not proved very satisfactory, partly because thebatteries and lamps involved are not sufficiently reliable. This isparticularly so for military field applications, where extremes oftemperature, moisture, shock and vibration etc. are encountered; e.g.,in the use of an Army rifle in jungle warfare. Such an arrangement isalso undesirably bulky and inconvenient to mount on the typical weapon.The use of a single spot of radio-luminescent material at the center ofa circular, transparent reticle has been considered as a means ofilluminating the rear sight of a rifle. This appears to present somedifficulties, partly because a halo of light floods the reticle,obstructing vision and also because the radio-luminescent spot is whitein color; furthermore, without illumination of the foreward sight, it isdifficult to properly align the weapon.

SUMMARY OF THE INVENTION The present invention contemplates usingradioluminescent segments, preferably comprising red phosphors, to markand define (at least a portion of) the sight markings (or criticaledges) in such sighting devices. These radio-luminescent markings willprovide adequate and variable (red) illumination of the sight markings,without interfering with the convenient use of the weapon. By using ared phosphor, the illumination can be brighter, providing betterdefinition to the markings or critical edges, without blinding theoperator or being visible to the target.

It will be evident that a primary object of the subject invention is tosolve at least some of the foregoing problems and provide at least someof the foregoing features and advantages. A related object is to provideoptical sighting devices with radio-luminescent segments defining atleast a portion of the sight markings.

A further object is to provide such radio-luminescent segments usingparticular isotopic materials which are sufficiently active to providefor adequate stimulation of the phosphorescent material, but which emita minimal amount of penetrating radiation that does not constitute ahealth hazard to the operator.

A related object is to provide such radio-luminescent segments usingparticular phosphorescent materials such as zinc sulfides, yttrium(europium) vanadates, gadolinium(europium) oxides, or like red phosphorsin order to provide maximum illumination of the sight markings orcritical edges without impairing the vision of the operator orpermitting the weapon to be located by the intended prey.

A further related object is to use such red phosphor materials incombination with Promethium-l47 (isotope) so as to properly activate thephosphor and provide prescribed illumination with a minimum ofpenetrating, exterior, ionizing radiation and with an adequatebrightness half-life associated with the illumination to provide forsatisfactory operation over a period of several years.

Other objects are to miniaturize sight-illuminating arrangements, toeliminate problems associated with portable battery lamps, light fibers,and other devices, and to optimize the use of radio-luminescentmaterials so that the aforedescribed problems are avoided.

How the foregoing and other more specific objects are achieved willappear and become evident through consideration of the ensuingdescription of preferred embodiments of the invention in conjunctionwith the associated drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a highly schematic isometric viewof an embodiment of the invention, with parts omitted or indicated onlyfunctionally for clarity.

FIG. 2 is a schematic isometric view of a rear sight embodiment likethat in FIG. 1 but modified somewhat.

FIG. 3 is a schematic isometric view of a front sight embodiment likethat in FIG. 1 but modified somewhat.

FIG. 4 is a rather simplified isometric view of a singleradio-luminescent marking segment modified and indicated asrotatableabout its own axis to make the emanating illumination variable inquality and/or intensity.

FIG. 5 is an upper perspective view of a radioluminescent tubeembodiment suitable for use in the arrangements of FIGS. 1 to 4.

FIG. 6 is a very schematic isometric view of other embodimentsillustrating means for selectively varying illumination quality andintensity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a suitableapplication environment for a preferred embodiment of the invention tobe described. Here, an optical sighting arrangement OSA is shown,comprising from and rear sights, 30 and 20, re-

spectively, that are shown mounted, through associated brackets 35 and25, respectively, upon a common support indicated in phantom as supportB which, for purposes of illustration, may be understood as includingthe frame portion of a rifle or like weapon supporting the opticalsighting device, being used by an operator for aiming the weapon in amanner understood in the art. Accordingly, sights 20 and 30 (beingfunctionally shown) may be understood as aligned along a common optical(sighting) axis SA, shown in phantom extending from the working end ororigin (operators eye) of the sighting device along, and through, thesighting arrangement to terminate at a selected target locus T. Thesighting arrangement is adapted, conventionally, to enable an operatorto aim the piece so as to hit a prescribed target zone (here indicatedas the center of target T). Axis SA thus defines the center of the fieldof view associated with the sighting arrangement, terminating at thecenter of illustrative target T.

Front sight 30 comprises a cylindrical radioluminescent tube 33 mountedin alignment with, and concentric along, axis SA upon a suitable arm 31which, in turn is attached to fixed bracket 35. Rear sight 20 comprisesa planar opaque supporting plate 21 of any suitable shape and dimensionas dictated by established sight design and accommodated upon thesupporting weapon. Plate 21 includes a relieved sighting-apertureportion 22. Aperture 22 preferably comprises a generally circularcut-out of a radius which defines a prescribed field of view that willsurround the target (taking into account the relative location of originO and other dimensions as is conventional in the art). The center 20-Cof aperture 22, lies along axis SA and forms the focus or origin fromwhich the upper edges of plate 21 and the pair of elongateradioluminescent tubes 23 and 24, mounted therealong, all extend.Preferably, these upper sighting-edges (critical edges) are bevelledalong a pair of symmetrical axes originating at focus 20-C. Sight 20 isrigidly affixed on bracket 25 which, in turn, is affixed upon support B.Brackets 25, 35 may of course (either or both) be adjustably affixed tosupport B so as to permit the repositioning and adjusted alignment ofthe associated sights, as is well known in the art. Moreover, thegeneral mechanical arrangements of the foregoing sighting device(including the general configuration of sight 20) are generally known inthe art.

Radio-luminescent tubes 23, 24, and 33 may be understood as, all,generally fashioned in the manner indicated in FIG. as tube 1 and arehereinafter described in some detail, although modified embodiments willalso serve. Radio-luminescent tube 1 comprises an outer casing 3enclosing and housing an inner slug 5 of radio-luminescent material.Tube 3 preferably comprises a closed cylinder. Radio-luminescent tube 1can also be constructed by suspending the fine-powder mixture ofradio-luminescent material in a monomer (or un-solidified form of anytransparent plastic material) and then fixing the radio-luminescentmaterial within the solidified transparent plastic material by castingit in the desired cylindrical shape. This cylindrical shape may also beachieved by extruding a rod of plastic material, containing thesuspended radio-luminescent material, as the plastic solidifies. Thensingle tubes would be sectioned from the extruded rod. It may bedesirable to coat the impregnated tubes" with a thin layer of plasticmaterial that does not contain radioluminescent material in order toprovide a surface from which radioactive powder cannot be released byabrasion or wear.

In the case of embodiments 23, 24, the tube is partially shielded withan opaque material so as to provide one or more translucent windowsections along at least a portion of the side walls so as to generatelinesegments of illumination along the tube length at selectedcircumferential locations thereof. Such a line of illumination can alsobe achieved as in the case of tubes 23 and 24 by embedding all but aportion of the side of the tube into the sight frame. Tube 3 is aboutone-quarter to one-half inches long, as required, providing thenecessary length of linear definition, with an inner diameter ofone-sixteenth to one-thirty-second inches and with a wall thickness asmay be required to provide adequate strength and integrity to the tube.

In the case of end-illuminated embodiment 33, the tube is asafore-described except, of course, that one end (only) is translucentwhile the other end and the sides are opaque to shield it opticallydirecting illumination only back along axis SA toward origin 0. In thecase of 33, the entire tube 1 can be embedded in a metal holder, etc.,exposing only the tube end to provide an illuminated dot along sightingaxis SA. Also, in the case of tube 33, the tube could be embeddedvertically in a vertical post-type sight (replacing 30), exposing onlythe curved end of the tube 1 to provide an illuminated shaft alongsighting axis SA.

The radio-luminescent materials comprising slug 5 may take a number offorms, all receiving an energizing isotope plus a luminescent materialresponsive thereto. We prefer to use phosphors of the zinc sulfide type,that emit illumination in the red region of the visible spectrum. Wehave found this particularly suitable for night, or otherlow-illumination, situations, the red illumination being less blindingto a darkness-adapted eye than other regions of the visible spectrum orwhite light of the same brightness. Other phosphors such as those basedupon rare-earth compounds, such as yttrium(europium) vandate,yttrium(europium) oxysulfide, or gadolinium (europium) oxide may be usedin certain situations instead of, or in combination with, the zincsulfide type.

Various radio-isotopes may be used as the activating energy source.Generally, the penetrating nature of gamma radiation and its associatedhealth-physics problems make gamma emitting isotopes unacceptable.Alpha-emitting isotopes may be used, but the energyconversion efficiencycan be quite low due to short mean-free path. Beta emitting isotopes aregenerally preferred. We have considered tritium, promethium (Pm-147),krypton (Kr-85), and strontium (Sr-90) primarily. Kr-85 and Sr-90,however, have the disadvantage of emitting considerable penetratinggamma radiation in addition to their beta activity, and thus present ahealth hazard. The beta energy of tritium is only 1/ 10 that of Pm-l47and accordingly much more must be rounding the operator is brighter (butstill insufficient to permit him to define his sights without the aid ofillumination), the sight illumination must be brighter to provideadequate definition. If, however, the environment is quite dark, (butstill sufficient to illuminate the target), it may be necessary toreduce the sight illumination from that required for the brighterenvironment to a lower level of sight illumination that will notinterfere with the ability of the operator to perceive him dimly-littarget.

Thus, a radio-luminescent tube 1 (analogous to tubes 23, 24, etc.) asindicated in FIG. 4 may be enclosed (to intercept useful illumination)by rotatable filter CF adapted to provide selectable levels of emanatingbrightness on the linear sight markings. Filter CF is adapted to berotated relative to tube 1' selectably interposing optical filters ofvaried opacity between the radio-luminescent phosphor source and theilluminated marking across the given field of view (angular field Fo Vindicated). A radio-luminescent tube 1' may be understood as constructedgenerally along the lines of tube 1 in FIG. 5, comprisingradio-luminescent slug 5' surrounded by transparent encapsulation tube 3(or as comprising radio-luminescent material suspended in transparentsolid plastic), which in turn is surrounded by filter CF (at leastacross field Fo V). Filter CF is provided with various strip portions orsegments S1, S2, S3, etc. of selected, different opacities as known inthe art and is made rotatable with respect to the enclosedradio-luminescent tube. Thus, for instance, when a given angular sectorof the tube is exposed across the field FoV (e.g. as defined by the maskprovided by plate 21' in FIGS. 2A, 28) then a prescribed, selectedlight-attenuating strip 8-3 of prescribed opacity will be interposed toset the level of illumination as selected by the operator. For adifferent illumination, the filter CF may be shifted (by mechanicalmeans known in the art but not shown) to interpose another filter stripof dif ferent corresponding opacity. The adjustable linear illuminationmay also be provided by coating the tube with segments of a transparentmaterial of varying opacity in each segment and then providing for theentire coated tube to be rotated, exposing the each segment to theunshielded portion of the mounting.

The means for mounting the luminescent tube and its surrounding filterso as to be rotatable with respect thereto in a given gunsightconfiguration will be apparent to those skilled in the art and will notbe dwelt upon here. Filter CF may, for instance, comprise an arcuatesegment of film spanning the length of the luminescent tube, with thestrips of varying opacity each being long enough to cover the exposedwindow (field Fo V) involved in the subject mounting arrangement.

A further modification for varying the emanating intensity is shown(very functionally) in FIG. 6 where an exemplary radio-luminescent tube10 (understood as along the lines of those described before and adaptedto emit light of a given original intensity from its exposed headportion not shown but generally indicated as light rays P focused atmarking spot (CH). Tube 10 is adapted to project radiant energy at agiven original intensity to be focused (by optics not shown, but wellunderstood in the art) onto a prescribed spot CH portion of a sightingscreen or reticle F. Spot Cl-I will, for instance be the functionalanalog of the spot of light projected by the end of tube 33 in theembodiment of FIG. 1 and adapted to define the center of the sightingaxis SA in the given sighting arrangement. Also shown is an intensityadjusting filter means F comprising a generally-circular disc having(quadrant) sections FS-l, FS-2, etc. of different selectable opacities,filter F being rotatable as indicated (e.g., by the operator) tointerpose one of the sections FS between radioluminescent source 10 andthe operators eye and thereby allow a selectable adjustment of theintensity of the light projected as spot CI-I. One may use a likearrangement employing a filter belt. Thus, it will be apparent thatwhere the embodiment involves a sighting spot CH on the gunsight of arifle used in low illumination situations (night fighting) for a targetT the rifleman/operator may rotate filter F to provide a sighting spotCH of an intensity selected to most closely suit the level of targetillumination at a particular time. Obviously, filter F may includevirtually any number of selectable filter sections having differentopacities andlor filtering characteristics (e.g., color) and may takethe form of other analogous devices for selectably attenuating the levelof illumination projected from the radio-luminescent source.

Those skilled in the involved art will readily understand that thesubject invention may take other analogous forms still falling withinthe spirit and scope of the subject invention as defined by the appendedclaims for instance, involving variations in structure, materials, orprocesses used. For instance, radio-luminescent devices of the typedescribed may obviously be used in defining the markings of othersighting or like arrangements while in certain cases, other phosphorsand/or other radio-isotope materials may in certain instances be apt forachieving the indicated results and performing at least some of thedescribed unique functions.

What is claimed is:

1. In a sighting arrangement including sight marking means andsupporting means, the arrangement being adapted to enable an operator toposition the supporting means so as to align the marking means along aprescribed sighting axis, extending between a point of origin and atarget point, this arrangement including an elongate chamber arrangedalong a portion of the axis wherein at least one reference point alongthe axis is to be indicated by one or more marking segments, theimprovement therein comprising:

. at least one illumination arrangement comprising a translucent housingcontaining radio-luminescent means adapted to project an illuminationbeam to delineate at least one of said marking segments, saidradio-luminescent means comprising a matrix of clear plastic material inwhich are relatively homogeneously suspended particles of aphosphorescent material emitting light in the visible red spectrum, andparticles of Promethium-l47 isotope 2. The combination recited in claim1 wherein said phosphorescent material comprises a zinc sulfide typephosphor.

3. The combination as recited in claim 1 wherein the sightingarrangement comprises a gunsight for a weapon.

4. An optical marking device for defining an alignment pattern underlow-illumination conditions comprising at least one radio-luminescentcapsule, this capsule comprising container means, luminescent meanscomprising a matrix of clear plastic material in which are relativelyhomogeneously suspended particles of 5; sorbed within ceramicmicrospheres, such as the synthetic zeolites, and the rinsed, loadedspheres then fired to fix Pm O inside, with surface activity thereafterbeing removed with an acid leach. Of course, in

using Pm-l47 it should be b orne in mind that lightin tensity wiT cayioliapproximately of the initial b is tness u about five years i 9 1 4 713 @2- year half-life.

Accordingly, with our preferred embodiment, indisated abov Pm-147 ist12. use binsdw a red-phosphorescent material. Preferably, a zinc-sulfidephosphor and the Pin-147 microspheres are mixed together, the sizes ofboth particles typically ranging from to 50 microns.

Using the foregoing mixture of promethiumimpregnated ceramicmicrospheres and zinc sulfide phosphor particles we have found thefollowing relat on be w n Put-14 on o an re u tant brightness, asmeasured in microlamberts to be illustrative of what may be expected:

TABLE I Pm-l47 Concentration Brightness" (mCi) (UL) milli-curics ofPromethium I47 brightness in microlamberts as compared to a white,certified standard phosphorescent light source brightness ola red zincsulfide phosphorescent system in an 6-inch glass tube, tG-inch long; allother values are for white zinc sulfide systems as circular flat spots,3/32-inch in diameter.

Of course, brightness can be varied from a few microlamberts (about theintensity of a watch dial) up to a few millilamberts for the whitesystem (the intensity of the typical television receivers cathode raytube is on the order of several hundred millilamberts; similarly fortypical military instrument dials, pushbuttons, etc.). We prefer to keepthe brightness of the red system to the order of 199 250 microlamberts,as measured with a photometer that has been calibrated with a similarzsz tk wn bri htne s s n a niqs. P-. 47. "l mspheres and a whitephosphor.

As a modification of the foregoing radio-luminescent composition, a redrare-earth phosphor may be impregnated in the microspheres along withthe P n-147 to ac liie ve ahigh er conversion efficiency. In most cases,the Pm-l47/red-phosphor mixture embedded in tiny ti'ansparent tubeswhich are, in turnfaffixed as marking segments" on the sight will be thepreferred embodiment because of their desirable performance inlow-illumination situations. For certain instances, one

can prepare an isotope-phosphor mixture of the type described inconjunction with a paint vehicle and simply spread it as a paintedmarking on the surface or critical edge of the sight. However, this willobviously expose the radio-luminescent mixture to risks of weatheringand associated deterioration, accidental abrasion and removal, etc.Thus, the encapsulation of the mixture sealed within a tiny transparentglass (or plastic, etc.) tube or suspended in a solid transparentplastic tube of the type described, exposing only the end or side of thetube to describe the desired configuration segment (i.e., theilluminated dot or line) avoids this problem.

The dimensions of the tube 1 (length, inside and outside diameters,etc.) may, of course, be adjusted as required by the specificconfiguration of the sight to be illuminated. In general, thefront-sight will be adapted to provide a spot source mark (or glowingdot) by exposing only the end of a single radio-luminescent tube. Therear-sight will typically require one or more linear radio-luminescentsegments (light sources; e.g., provided in the embodiment by exposingthe side of tube 1) arranged to form a Vee", a U, a square, or similargeometries, within which the front spot is to be positioned (alignedwith) during a typical sighting routine.

FIGS. 2 and 3 illustrate rear and front rifle sights, 20, 30respectively along the lines of the embodiment in FIG. 1 but slightlymodified and, of course, shown in somewhat greater detail and inenlarged scale. Thus, rear sight 20' is generally the same as sight 20in FIG. 1 (except where indicated) and includes a cut-out (aperture) 22'in an opaque plate 21' with a pair of opposed, similarradio-luminescent-tube-receiving slots 24-8, 23-8 flanking aperture 22,each slot being aligned along (parallel to) the upper edges of sight 20,these edges being bevelled to comprise a V configuration, the arms ofwhich originate from associated optical axis SA (as in the embodiment ofFIG. 1). Plate 21' may assume any convenient dimensions (e.g., on theorder of a few eighths of an inch square by a few sixteenths inthickness, with the radius of slots 24-8, 23-8 and cut-out 22' being afew sixty-fourths).

Front sight 30 is also generally the same as sight 30 in the embodimentof FIG. 1 except where specified, and includes a holder 31' (ourmounting device) including a hole 33-8 for receiving the associatedradioluminescent-tube. Mount 31 will be assumed as formed of metal,opaque plastic or the like and adapted to mask out any stray light fromthe glowing radioluminescent tube in hole 33-8, except for thelight-spot projected back along optical axis SA toward rearsight 20 (aswith the embodiment of FIG. 1). Holder 31' will be suitably dimensioned(e.g., cross-sectional dimensions of several thirty-seconds inch with aradius being cut-out across its base to provide for its being mounted onthe barrel of the weapon.

INTENSITY ADJUST According to a further improvement and feature ofnovelty, the radio-luminescent tubes may be made adjustable inbrightness (intensity of illumination). For example, this allows anoperator to adjust brightness of the illuminated segment markings of hissighting device to suit the lighting conditions of his environment andrelative to the brightness of the target T within his field of view at aparticular time. If the environment surof said container means beingsufficiently translucent to emit the requisite luminescence to definethe said pattern appropriately for the conditions prevailing at the timeof regulating the shielding, whereby durable utility for the markingdevice is attainable not merely throughout a variety of illuminationconditions but also after significant decay of the radioisotope.

1. In a sighting arrangement including sight marking means andsupporting means, the arrangement being adapted to enable an operator toposition the supporting means so as to align the marking means along aprescribed sighting axis, extending between a point of origin and atarget point, this arrangement including an elongate chamber arrangedalong a portion of the axis wherein at least one reference point alongthe axis is to be indicated by one or more marking segments, theimprovement therein comprising: at least one illumination arrangementcomprising a translucent housing containing radio-luminescent meansadapted to project an illumination beam to delineate at least one ofsaid marking segments, said radio-luminescent means comprising a matrixof clear plastic material in which are relatively homogeneouslysuspended particles of a phosphorescent material emitting light in thevisIble red spectrum, and particles of Promethium-147 isotope materialas an activating radio-isotope material.
 2. The combination recited inclaim 1 wherein said phosphorescent material comprises a zinc sulfidetype phosphor.
 3. The combination as recited in claim 1 wherein thesighting arrangement comprises a gunsight for a weapon.
 4. An opticalmarking device for defining an ''''alignment pattern'''' underlow-illumination conditions comprising at least one radio-luminescentcapsule, this capsule comprising container means, luminescent meanscomprising a matrix of clear plastic material in which are relativelyhomogeneously suspended particles of phosphorescent material andparticles of Promethium-147 sealed within said container means toprovide initially a surplus amount of luminescence emitting light richin the visible red spectrum and instantly regulatable shielding meansfor masking out a portion of said luminescence to define said pattern,at least a portion of said container means being sufficientlytranslucent to emit the requisite luminescence to define the saidpattern appropriately for the conditions prevailing at the time ofregulating the shielding, whereby durable utility for the marking deviceis attainable not merely throughout a variety of illumination conditionsbut also after significant decay of the radioisotope.